1. Field of the Invention
The present invention relates to a semiconductor laser element and a method of manufacturing the same, and particularly to a method of dividing a wafer having a semiconductor structure on a substrate into individual semiconductor laser elements.
2. Discussion of the Related Art
Generally, in a semiconductor laser element, the resonator surface for laser oscillation is formed by using cleaved surfaces of a crystal. Because an AlGaInN-based group-III nitride semiconductor has a hexagonal crystal structure, in a step of forming resonator surfaces (hereinafter may be referred to as “step of obtaining laser bars”) by dividing a semiconductor wafer (hereinafter may be referred to as “wafer”) by cleavage, the dividing direction may deviate from its intended direction. In order to prevent the deviation in the dividing direction, for example, a technology for forming cleavage guide grooves 113 in shape of broken lines along the dividing direction which is perpendicular to the optical waveguides 112 in a LD structure 111 stacked on a GaN substrate 110, as shown in FIG. 6, is disclosed (for example, see Japanese Laid-Open Patent Application Publication No. 2009-105466 A).
However, as in the above mentioned publication, forming of the cleavage guide grooves by using RIE (Reactive Ion Etching) takes time which results in decreasing of productivity. For this reason, a technique is proposed in which on a lower surface of a wafer on which the semiconductor layer 1 is stacked and a ridge 4 is formed on the semiconductor layer, as shown in a cross sectional view of FIG. 7(a), a cleavage guide groove 5 is formed and dividing is carried out. Examples of techniques for forming the cleavage guide groove 5 in a short time include a processing technique using a laser beam. However, a processing technique using laser irradiation has a problem that since the formation is conducted using a high energy for a short time, microscopic irregularities occur on the surfaces of the cleavage guide grooves 5. Cleaving a wafer 1 along such cleavage guide grooves 5 results in generation of line-shaped step differences 3 (in the present specification, the term “line-shaped step differences” may be referred to simply as “lines” or “step differences”) due to deviation in height of the cleaved surface with respect to the direction perpendicular to the cleaved surface 2, which is caused by the microscopic irregularities on the surfaces of the cleavage guide grooves 5. That is, the step difference 3 occurs as shown in FIG. 7(b) when viewed from the upper surface of the wafer (a direction shown by an arrow A in FIG. 7(a)), and as shown in FIG. 7(c) when viewed from a side surface of the wafer (a direction shown by an arrow B in FIG. 7(a)). If the step differences in the cleaved surface reach the optical waveguide, light oscillating in the optical waveguide at the time of operating the laser hits the step difference and is scattered, which results in decrease in the optical output power and emission in unintended directions, which adversely affect the FFP (Far Field Pattern).